1. Technical Field
The present invention relates to an apparatus for pretensioning seat belt webbing which is extensible about an occupant of a vehicle seat.
2. Description of the Prior Art
It is known to pretension seat belt webbing in the event of a vehicle collision. In one known seat belt webbing system, the seat belt webbing is pretensioned by moving a buckle associated with the seat belt webbing through a pretensioning stroke. The pretensioning stroke is accomplished by actuating a pyrotechnic charge in a piston/cylinder device.
It is characteristic of the known pretensioning devices that the final pretensioning force which is applied to the occupant (due to tension in the seat belt webbing) is dependent on the amount of slack in the seat belt webbing which is removed during the pretensioning stroke. The amount of slack in the seat belt webbing varies with the size and shape of the occupant of the seat as well as with the type of clothing worn by the occupant and the design of the vehicle seat. Due to an inherently fast pressure drop in the pretensioner cylinder after actuation of the pyrotechnic charge and the variation in the amount of slack in the webbing, the amount of the final pretensioning force can vary for occupants of different sizes and shapes.